In MOS technology it has proven economical to incorporate as many functions as possible into a single semiconductor chip thereby to reduce dependence upon external components. Unfortunately even the best MOS processes result in substantial variation in device parameters. Accordingly, in circuits where performance is related to particular device parameters, either the critical circuits are manufactured external to the MOS chip or some form of external compensation is applied. Alternatively extremely careful process controls must be used in manufacturing if low chip yield is to be avoided. Such controls are expensive and are particularly undesirable where the critical circuit involves only a small portion of the MOS circuit.
Voltage sense amplifier circuits have proven particularly difficult to integrate into MOS chips. In such a circuit an output is made to change from one level to another as the input varies past a particular, or sense, level. Thus, in effect, an analog to digital conversion is employed. The output is digital, either a one or a zero, whereas the input is analog in that it can be any value. When such amplifiers are manufactured in MOS form, it is found that the input level, at which the output switches between digital states, will vary as a function of MOS process variables. Thus the voltage being sensed is not a reliable absolute value as is normally desired.